Generally, twisted nematic liquid crystal displays (TN-LCDs) have been used as thin film transistor liquid crystal displays (TFT-LCDs). In TFT-LCD display devices, liquid crystals having dielectric anisotropy are injected between glass substrates and transparent electrodes are deposited on the glass substrates. The liquid crystals are aligned to twist continuously at 90 degrees against the surfaces of the upper and lower substrates. Accordingly, light, which is incident on the substrates, passes through the substrates, rotating at 90 degrees according to the twisted orientation of the liquid crystals. As will be understood by those skilled in the art, polarizers and analyzers are typically attached on the upper and the lower substrates, respectively, to use the above-mentioned twisted operation to change the amount of light transmission. However, a plane viewing angle is typically limited since the liquid crystals are twisted within a range of 90 degrees. To overcome the above-mentioned disadvantages, attempts have been made to develop in-plane switching (IPS) TFT-LCD display devices.
In the IPS TFT-LCD display devices, the maximum viewing angle is improved since a common electrode and a pixel electrode are formed on the same substrate, whereby the liquid crystals operate only in a direction which is parallel to the substrate. However, the aperture ratio is typically reduced to half since the opaque pixel electrode and the common electrode are formed side-by-side on the substrates. To compensate for the reduced aperture ratio, the brightness of the display's backlight should be strong to compensate for the reduced transmittivity. However, as will be understood by those skilled in the art, to reduce leakage currents caused by the higher light intensity, it is typically preferable to use channel protection type TFTs having thin semiconductor layers rather than channel etch type TFTs having thick semiconductor layers.
Hereinafter, the structure of a conventional channel protection type IPS TFT-LCD display device and a fabrication method therefor is explained in detail with reference to the accompanying drawings. In particular, FIG. 1 is a schematic layout diagram of a display device according to the prior art. As illustrated by FIG. 1, a plurality of gate lines 1 are formed in parallel on a substrate. A plurality of data lines 2 are also formed and these data lines 2 extend orthogonal to the gate lines 1. Pads 1' and 2' are formed at end portions of the gate and the data lines 1 and 2 and provide means for receiving a scanning signal or a pixel signal from an external driving circuit and transmitting these signals to the gate and data lines 1 and 2.
FIGS. 2A-6C illustrate a method of fabricating an IPS TFT-LCD display device according to the prior art. In particular, FIGS. 2A, 3A, 4A, 5A and 6A are layout schematic views of intermediate structures which illustrate a method of forming a TFT-LCD display device according to the prior art. FIGS. 2B, 3B, 4B, 5B and 6B are cross-sectional views of the intermediate structures of FIGS. 2A, 3A, 4A, 5A and 6A taken along line A-A' and FIGS. 2C, 3C, 4C, 5C and 6C are cross-sectional views of intermediate structures which illustrate a prior art method of forming a gate pad of a TFT-LCD display device. Referring to FIGS. 2A-2C, a metal layer such as Al, Al--Nd, Al--W or Al--Ta is deposited on a substrate 10 to a thickness of 200 nm, and patterned to form a gate electrode 11, a common electrode 15, a common electrode line 18 and a gate pad (reference number 1' in FIG. 2C). Next, a gate insulating film (reference number 12 in FIGS. 2B and 2C), an amorphous silicon layer (reference numeral 13 in FIGS. 2B and 2C) and a channel protection film (reference numeral 14 in FIGS. 2B and 2C) are successively deposited. The gate insulating film 12 is formed of a silicon nitride (SiNx) layer to a thickness of about 200-400 nm, the amorphous silicon layer 13 is formed to a thickness of less than 50 nm, and the channel protection film 14 is also formed of silicon nitride, an insulating material. The common electrode 15 is extended from the common electrode line 18, and the gate line 1 is formed in parallel to the common electrode line 18. A part of the gate line 1 is extended to form the gate electrode 11.
The next step is illustrated in FIGS. 3A-3C. The channel protection film 14, which is made of silicon nitride, is formed through a photolithography process which only leaves a pattern above the gate electrode 11 but with a width less than the width of the gate electrode 11. When etching the channel protection film 14, a chemical material having selective etching characteristics between the channel protection film 14 of silicon nitride and the amorphous silicon layer 13 is used so as not to etch the amorphous silicon layer 13. After being etched through the photolithography process, the channel protection film 14 is formed within the border of the gate electrode 11. Here, the channel protection film 14, which is formed on the gate pad (reference numeral 1' in FIG. 3C), is completely removed. After this process, the process steps illustrated in FIGS. 4A-4C are performed. Referring to FIGS. 4A-4B, the amorphous silicon layer 13 is patterned and etched. Generally, the width of the amorphous silicon layer 13 is formed wider than that of the gate electrode 11. Referring to FIG. 4A, the width of the amorphous silicon layer 13 is formed horizontally larger than that of the gate electrode 11. In this process, the amorphous silicon layer 13, which is covered on the gate pad (reference numeral 1' in FIG. 4C) is completely removed.
Referring to FIGS. 5A-5C, an n.sup.+ amorphous silicon layer 16 is deposited to a thickness of about 50 nm and a metal layer such as Cr, Al or an Al alloy is deposited on the n.sup.+ amorphous silicon layer 16 to a thickness of about 100-400 nm. Both layers are successively etched through the photolithography process to form a data line 2, a pad 2', source and drain electrode (S and D) and a pixel electrode (reference numeral 17 FIGS. 5A and 5B). The amorphous silicon layer 13 above the gate electrode 11 is formed wider than the gate electrode 11, and the channel protection film 14 is formed on the amorphous silicon layer 13, but is narrower than the gate electrode 11. In addition, the data line 2 is formed to cross the gate line 1 and the common electrode line 18, and a part of the source electrode S is extended from the data line 2 to overlap a side of the gate electrode 11.
One terminal of the pixel electrode 17 overlaps the other border of the gate electrode 11 to form a drain electrode D, and the rest is extended toward the common electrode line 18, in parallel with the common electrode 15. In this process, only the gate insulating film 12 is left on the gate pad 1' since the n.sup.+ amorphous silicon layer 16 and the metal layer are completely removed during the step of etching the data line 2 and the pixel electrode 17 through the photolithography process.
Referring now to FIGS. 6A-6C, part of the insulating film 12 is etched from the pad 1' so that the pad 1' can be electrically connected to an external diving circuit. At least five (5) photolithography process steps are required to form the conventional TFT substrate having the above-mentioned structure. It is preferable that the number of photolithography process steps be reduced to decrease the cost and time to manufacture the TFT substrate since the photolithography equipment, photoresist and developing solution are expensive and additional process steps add to the time required to produce the TFT substrate. In addition, the alignment is performed by depositing the polyimide alignment film on the common electrode, which is under the insulating film, in the wiring of the TFT according to the conventional fabrication method. However, DC electric field components, which remain between the common electrode and the polyimide when the liquid crystal is driven by AC, are present because the polyimide film becomes ionized during operation. Accordingly, the conventional TFT is limited by a parasitic phenomenon commonly referred to as "image-sticking". Thus, notwithstanding the above described method of forming IPS TFT-LCD display devices, there continues to be a need for improved methods of forming display devices.